1. Field of the Invention
The present invention is directed to rigid polyurethanes and pour-behind-fabric, fiber-reinforced applications thereof. Particularly, the present invention pertains to cellular rigid polyurethanes containing aliphatic dibasic acid esters wherein such polyurethanes are especially useful in low density pour-behind fiber reinforced reaction-injection-molded systems and the resulting products known as structural-RIM or SRIM.
2. Description of the Related Art
Reaction-injection-molding (RIM) systems by now are well known to those skilled in the art. Commercial systems produce elastomeric products which contain polyurethane-polyurea linkages having many uses. However, such systems have relatively low heat distortion temperatures and lack the flex modulus and tensile strength necessary for many applications. Such systems are disclosed, by way of example, in U.S. Pat. No. 4,218,543.
The flex modulus and tensile strength of traditional RIM systems may be improved through the addition of short fiber reinforcement (RRIM). However, the addition of chopped fibers raises the viscosity of the system components as well as affecting the ease of processing. Moreover, this addition of reinforcement has little effect on matrix dominated physical properties such as heat distortion temperature (HDT).
The use of woven or non-woven (random) fiber reinforcement (Structural RIM or SRIM) can provide parts with yet greater tensile strength and flex modulus. Furthermore, the physical properties may be made directional through the use of unidirectional fibers or combinations of both unidirectional and random fiber orientations. Unfortunately, the use of such reinforcement has not proven practical with traditional RIM systems due to the high viscosities of such systems.
Alternative systems such as those described in U.S. Pat. Nos. 4,794,129; 4,935,460; and U.S. Pat. No. 4,983,659 issued to ICI Americas, Inc. disclose modifications to the reaction system which allow said systems' chemical reactivities to be tailored to a particular need. However, in addition to the formulation characteristics pour-behind-fabric SRIM applications also require open celled compositions which generally necessitates the use of a cell opening additive. Unfortunately, use of conventional cell openers, particularly silicone containing surfactants reduce the adhesion of the urethane to fabric in pour-behind-fabric applications and are therefore commercially unacceptable.
3. Summary of the Invention
According to the present invention it has been found that the addition of a dibasic acid ester (DBE) selected from the group comprising dimethylesters of adipic acid, glutaric acid, succinic acid, and mixtures thereof to a polyurethane formulation function effectively as a cell opening agent without affecting the adhesion between the polyurethane substrate and the surfacing film or fabric. For the purposes of the present invention it is understood that the polyurethane formulations also contain polyisocyanurate linkages.
Furthermore, DBE's are low viscosity materials that when added to conventional urethane systems, reduce viscosity and improve the flow of the resulting urethane formulation through reinforcing materials, thereby obviating the need for chemical modification or reactants such as those disclosed in the ICI patents mentioned above. When using the DBE's in accordance with the present invention there is no need for further addition of organo-silicon containing compounds as disclosed in U.S. Pat. No. 3,884,848.